FIG. 4 is a circuit diagram showing the configuration of a conventional resonant switching power supply circuit.
In FIG. 4, an AC rectifying section 103 is connected to a commercial power supply 101 via a noise filter 102. The rectified output from this AC rectifying section 103 is smoothed using a smoothing capacitor 104. The DC output obtained using the smoothing capacitor 104 is supplied to a switching circuit 105 having switching elements 105A and 105B that perform switching operations. The output of this switching circuit 105 is supplied to a resonance circuit 106 comprising a series connection of the primary winding 107A of a transformer 107 and a resonance capacitor 106A. A rectifying/smoothing circuit 109 comprising diodes 108A and 108B and a capacitor 110 is connected to the secondary winding 107B of the transformer 107 described above. This rectifying/smoothing circuit 109 is connected to a control circuit 120 for controlling the switching operation of the switching circuit 105 described above via a voltage feedback circuit 113, and has output terminals 112A and 112B to supply a DC output to the outside.
The voltage feedback circuit 113 is connected to an external control terminal 124 via a resistor 115. The control signal from this external control terminal 124 is input to the voltage feedback circuit 113, whereby the resonant switching power supply device is adjusted so that a predetermined DC output voltage is obtained.
Usually, the turn ratio of the transformer is determined so that the secondary output voltage of the transformer becomes the desired minimum voltage. When an attempt is made to stably lower the minimum output DC voltage of the resonant switching power supply device, it is necessary to increase the number of turns of the primary winding of the transformer.
In the conventional resonant switching power supply device configured as described above, the voltage output from the rectifying/smoothing circuit 109 via the output terminals 112A and 112B is input to a voltage comparator 113A inside the voltage feedback circuit 113, and compared with a reference voltage Vref. The error voltage obtained by the comparison with the reference voltage Vref using the voltage comparator 113A is input to the control circuit 120 via a photocoupler 113B. The control output of the control circuit 120 is applied to the switching elements 105A and 105B of the switching circuit 105, thereby changing the switching frequency. The device is configured so that the control voltage obtained by changing the output voltage and the control signal obtained in response to the input voltage fluctuation and load fluctuation from the commercial power supply 101 are input to the external control terminal 124. The conventional resonant switching power supply device carries out control so that the predetermined DC output voltage is generated, using the control voltage and the control signal being input to the external control terminal 124.
Furthermore, a conventional resonant switching power supply device that detects a load current and controls the leakage inductance of its converter transformer in response to the load current is disclosed, for example, in Japanese Patent Application Laid-open No. Hei 9-163735. PATENT DOCUMENT 1 Japanese Patent Application Laid-open No. Hei 9-163735